The invention relates to a process for the homogeneously catalyzed acetic acid pyrolysis for the preparation of ketene and/or secondary products.
The homogeneously catalyzed pyrolysis of acetic acid is carried out on an industrial scale for the production of acetic anhydride, diketene and ketene. Ketene is further processed directly to form secondary products, e.g. sorbic acid and dimethylacrylolactone. The reaction proceeds at elevated temperatures in the range from 400 to 800xc2x0 C. and at reduced pressure. In addition to the principal products water and ketene, under said reaction conditions, a series of by-products are formed, e.g. carbon monoxide, carbon dioxide, ethene, ethyne, methane, propadiene and hydrogen. After the reaction, in the further course of the process, unreacted acetic acid and water are condensed out and the process gases are scrubbed with acetic anhydride. The following process steps depend on the target product.
Hitherto, only phosphoric acid derivatives having a defined vapor pressure, e.g. liquid triethyl phosphate having a boiling point of 215xc2x0 C., have been used on an industrial scale for the catalysis of the process. These are comparatively expensive, however. DE 687 065 does disclose using phosphoric acid as a catalyst, but only under the conditions of heterogeneous catalysis and on a laboratory scale.
The heterogeneous processes proposed in the older patent literature have not been able to establish themselves in industrial practice, since the ketene yields are much lower in comparison with homogeneous catalysis, e.g. using triethyl phosphate, and catalyst beds of phosphoric acid derivatives and phosphates are rapidly deactivated under the reaction conditions by the coke coating and led to a blockage of the reactor.
The object underlying the invention, therefore, was to improve the process mentioned at the outset by identifying cheaper homogeneous catalysts having the same or better catalytic action as triethyl phosphate.
It has now surprisingly been found that the same ketene selectivity and the same acetic acid conversion rate in comparison with triethyl phosphate can be obtained if, as catalyst, phosphoric acid is sprayed in liquid form into the acetic acid vapor.
This contradicts previous studies and literature data, according to which the catalytic activity of triethyl phosphate is said to be significantly better and is all the more surprising since phosphoric acid is considered to be of low volatility, has no defined boiling point and, on evaporation, higher-molecular weight condensation products are formed with elimination of water. The condensation products of phosphoric acid (e.g. metaphosphoric acid and phosphorus oxides) formed at high temperature are expected to block the plants.
The invention therefore relates to a process for the homogeneously catalyzed acetic acid pyrolysis for preparing ketene and/or secondary products, which comprises spraying, as catalyst, phosphoric acid in the form of a liquid jet into the acetic acid vapor.
The term phosphoric acid in the context of the invention includes all oxygen acids of phosphorus and solutions of their anhydrides, in each case individually or in a mixture. Preference is given to orthophosphoric acid, solutions of metaphosphoric acids, oligomers of phosphoric acid and phosphorus pentoxide. The molar content of phosphoric acid is preferably 100 to 10,000 ppm, calculated as the molar content of elemental phosphorus (P), based on the amount of pure acetic acid (CH3COOH) used in total.
Particular embodiments are given by the subclaims.
The phosphoric acid is preferably sprayed into the acetic acid vapor in the liquid state as commercial phosphoric acid (75, 80 or 85%) or diluted at elevated temperature. It can be added diluted with inorganic or organic solvents, preferably with water, methanol, ethanol, acetic anhydride or acetone.
In a preferred embodiment, the phosphoric acid is diluted with water or acetic acid in a ratio up to at most 1:100 and is sprayed at temperatures above 400xc2x0 C. as a continuous liquid jet at high impulse in the acetic acid main stream. That means that the velocity of the liquid jet is at least 10 times higher than that of the acetic acid main stream. This avoids contact of the liquid jet with the walls. As a result of the high temperature and good mixing with the preheated acetic acid, the phosphoric acid is successfully evaporated as rapidly as possible and virtually completely. Under these conditions, the same catalytic activity of phosphoric acid and triethyl phosphate was observed. Solid deposits in the area of the catalyst admixture and in the reactor did not occur during this.
In further preferred embodiments, the catalyst is added with addition of gas, preferably with nitrogen or ammonia, and/or it is added via nozzles, preferably via ultrasonic nozzles or two-component nozzles.